Nickel laterite ore bodies typically have a number of layers. For example, the ore body may have a layer of overburden with a low nickel content e.g. <0.8% nickel, which is usually discarded. Under the layer of overburden may be a limonite ore zone, a saprolite ore zone and then a layer of rock. It is to be understood that the thickness and the composition of the limonite and saprolite zones may vary widely between different ore bodies. As examples of the composition of the layers, the limonitic zone may contain about 1.2-1.7% nickel, about 40% iron and 1-4% magnesium oxide and the saprolitic zone may contain about 1.6-2.3% nickel and 7-25% iron.
Ores from the saprolitic zone are frequently treated using pyrometallurgical processes to recover ferronickel and a matte with 25-75% nickel content. Most of the ores from the saprolitic zone contain less than 20% iron, and the recovery of nickel from this zone may exceed 92%. More than 80% of the iron values are essentially lost in the slag. In some instances, ores from the limonitic zone are stockpiled, preference in recovery of nickel being given to the ores with higher nickel content found in saprolitic zones.
High pressure sulphuric acid processes have been developed to treat high iron content laterite ores. The amount of sulphuric acid required is dependent on the magnesium oxide content of the ores, but is often in the range of 30-40% by weight of the ore. In practice, the cost of sulphuric acid has been low enough to make extraction with sulphuric acid economically viable, but there are expectations that the cost of sulphuric acid will rise. In this process, no attempts are made to recover the iron values which are lost in the leach residue. This results in large volumes of leach residues, including gypsum produced in the process, and disposal is a major environmental challenge. Alternate processes that do not utilize sulphuric acid, with its potential environmental issues, are of interest.
The atmospheric leaching of nickel laterite ores using chloride and bio-technologies has been discussed by R. G. McDonald and B. I. Whittington in Hydrometallurgy 91 (2008) pp 56-69. U.S. Pat. No. 7,329,396 of G. B. Harris, V. I. Lakshmanan and R Sridhar, issued 2008 Feb. 12, describes the leaching of laterite ores using a lixiviant of hydrochloric acid and magnesium chloride. Canadian Patent 1013576 of H. F. Bakker, M. C. E. Bell and R. Sridhar, issued 1977 Jul. 12 describes selectively reducing particulate oxide material, exposing moistened reduced ore to a chlorine-containing gas and leaching with water. Canadian 1023560 of H. F. Bakker and R. Sridhar, issued 1978 Jan. 3, describes selectively reducing particulate oxide material, exposing moistened reduced ore to aqueous hydrochloric acid in an amount of less than 40% of the amount to be solubilized and water leaching under oxidizing conditions. Canadian Patent 1023952 of H. F. Bakker and R. Sridhar, issued 1978 Jan. 10, describes selectively reducing particulate oxide material, exposing moistened reduced ore to sulphur dioxide and leaching with water.
There is a need for processes to extract nickel and other value metals, and also iron, from laterite ores with improved yields of nickel and which have the potential for reduced problems in recovery of value metals and reduced environmental issues with most of the values being recovered as product. In particular, there is a need to separate iron from leachate solution prior to steps to recover value metals e.g. nickel and cobalt, produce an iron product and recycle the chloride which is mainly with the iron. The presence of iron in separation steps tends to lead to formation of sludges, loss of value metals by occlusion in the sludges, environmental issues in the disposal of the sludges and difficulties in recovery and recycle of components of the lixiviant used in the leaching process.